The invention described herein relates to a single-step production method for nano-sized cocrystals of explosives, and more specifically, a method capable of converting the desired coformer precursors to cocrystals with a mean crystal size in the nanoscale regime.
A compelling need exists to reduce the sensitivity of energetic materials so that accidental detonations from undesired stimuli such as shock and impact are minimized. This is particularly true for more powerful and generally more sensitive high explosives (HEs). One of the strategies for retaining the performance of these explosives while significantly reducing their sensitivity is to combine the energetic species into cocrystals having physical and chemical properties that are distinguishable from the pure species alone. A cocrystal is generated by combining significant quantities (to exclude cases where one material's presence is essentially a defect in the other material) of two or more coformers through chemical or mechanical means into one crystal structure. The hybrid crystals are unique crystal forms of well-known explosive molecules, possessing novel properties in comparison to the crystalline forms of the individual coformers which constitute them.
One practical application for cocrystals is for use in booster explosives, which must have a sufficient energy output to reliably initiate the newer, relatively insensitive main charge explosive fills, while exhibiting an acceptable level of sensitivity to unintended stimuli. Most existing booster high explosive (HE) formulations have unacceptable levels of sensitivity, thereby increasing the vulnerability of the entire munition to accidental initiation. Cocrystals of these HE formulations having reduced sensitivity while retaining the explosive power of their constituent materials would address these limitations.
Energetic materials such as 2, 4, 6, 8, 10, 12-hexanitro-2, 4, 6, 8, 10, 12-hexaazaisowurtzitane (CL-20) and 1, 3, 5, 7,-tetranitro-1, 3, 5, 7-tetrazocine (HMX) are examples of known high explosives having great explosive performance. CL-20, however, has not been widely used because it is more sensitive, i.e. more readily detonates in comparison to other secondary explosives. HMX is a state of the art explosive having one of the highest detonation velocities in the military. Both explosives are insoluble in water but highly soluble in organic solvents.
Cocrystals of CL-20 and HMX were previously synthesized and reported by Bolton et al., “High Power Explosive with Good Sensitivity: A 2:1 Cocrystal of CL-20:HMX” Cryst. Growth. Des., 2012, 12, 4311-4314 and Anderson et al., “Preparation of an Energetic-Energetic Cocrystal using Resonant Acoustic Mixing” Propellants Explos. Pyrotech. 2010, 35, 1-5. Bolton described a solvent based process to create HMX:CL-20 cocrystals, whereby HMX and CL-20 are dissolved in 2-propanol solution and precipitated from the solution. Anderson discussed using solvent drop and resonant acoustic mixing (“RAM”), whereby low frequency, high intensity acoustic energy is applied to the mixing vehicle along with very small amounts of solvent to mechanically mix HMX and CL-20 together until they form a cocrystal.
These solvent based methods, however, often result in impurities or unconverted crystals of the coformer(s) in the final product. Furthermore, these methods of making cocrystals are also relatively difficult to scale.
Nano-sized (less than 1 μm) cocrystals are possibly less sensitive than their counterparts with larger particle size. There have been reports that improved performance characteristics are associated with reducing the size of crystals. For example, the detonation failure diameter, also referred to as the critical diameter, is known to shrink with decreasing crystal size. In addition, HEs with a rounded morphology in plastic bonded explosives were found to produce less sensitive materials. Therefore, a need exists for a safe and simple manufacturing process to synthesize nano-sized cocrystals of energetic materials having improved sensitivity and reactivity.
Use of bead milling was previously discussed by Sowa et al in EP2751085A1 and Chiodo et al in WO 2013143927A1 to prepare cocrystals of fungicide and herbicides respectively. The bead mill has been shown to be an effective method of creating small particle size explosive materials safely using aqueous suspension (see P. Redner et al. “Production and Characterization of Nano-RDX” DTIC report, 2006). These methods, however, fail to disclose bead milling of at least two different explosives precursors having different solubilities in a liquid medium to prepare cocyrstals under a single-step process.